# [V](http://github.com/DJMcMayhem/V), 14 bytes

 òYp$xhòjòÄ$xhh

[Try it online!](http://v.tryitonline.net/#code=w7JZcCR4aMOyasOyw4QkeGho&input=MDEyMzQ1Njc4OQowMjQ2OA)

Explanation:

 ò ò "Recursively:
 Yp " Yank the current line and paste it
 $ " Move to the end of the current line
 x " Delete one character
 h " Move One character to the right.
 " Because of the way loops work in V, this will throw an error if there
 " Is only one character on the current line.

Now, the buffer looks like this:

 0123456789
 012345678
 01234567
 0123456
 012345
 01234
 0123
 012
 01
 0

We just need to do the same thing in reverse for the next line:

 j "Move down one line
 ò ò "Recursively (The second ò is implicit)
 Ä " Duplicate this line up
 $ " Move to the end of the current line
 x " Delete one character
 hh " Move two characters to the right.
 " Because of the way loops work in V, this will throw an error if there
 " Is only two characters on the current line.

[More interesting alternate solution](http://v.tryitonline.net/#code=w7LDhCR4aMOyw6deLzptMApkZEdwQHFk&input=MDEyMzQ1Njc4OQowMjQ2OA):

 òÄ$xhòç^/:m0
 ddGp@qd